This invention relates to a composition for the manufacture of an opaque silvery coating on a glass surface, particularly on a lamp, and a method for manufacturing such an opaque silvery coating.
It is known in the art to apply an opaque coating to various parts of lamps having a glass envelope. A typical example is the covering of the tip of an automotive lamp, in order to prevent the emission of direct (non-reflected) light beams from the headlamp of a vehicle. The light emitted by a lamp in a vehicle headlight is projected onto the road in a controlled manner, so as to avoid the blinding of other drivers. Therefore, the light being emitted forward from the lamp directly, and which would not be controlled by the projection optics, is prevented from escaping the lamp by a metal cover shield, or by an opaque layer or coating applied directly to the lamp surface.
The use of such an opaque layer is described, among others, in U.S. Pat. No. 4,305,016 and No. 6,270,237. For example, U.S. Pat. No. 4,305,016 discloses that a suitable composition for forming the opaque layer is accomplished by mixing carbonyl iron and silicon powder, and further forming a suspension with butanol as a carrier liquid. The suspension is dried, and the metal powder mixture is burned to form a stable, hard and opaque coating. However, this coating has a substantially black color. Recently, the need has appeared to provide a silvery coating on the lamps instead of a black coating, primarily due to aesthetical reasons.
In order to satisfy this need, it is proposed to employ a dual layer on the lamp in U.S. Pat. No. 6,270,237. A metal layer providing a silvery appearance is suggested to be made of various metals, for example Ag, Al, Ni, etc. and mixtures thereof. Another layer with light-absorbing properties is placed between the metal layer and the glass. This layer is essentially black, and may be manufactured from Fe, Mn, Ti or Si. According to the disclosure in U.S. Pat. No. 6,270,237, the purpose of this light absorbing layer is to prevent the reflection of light from the metal layer toward the reflector surface. This arrangement has the disadvantage that the manufacturing steps required for the additional layer makes the production more expensive.
Therefore, it is desirable to provide a composition, which could be applied in a single layer only, and which would ensure the required properties of the opaque layer, namely a silvery appearance, sufficient opaqueness, small backscatter, combined with long term stability and ease of manufacture. Further, it is desirable to provide an appropriate method for manufacturing such an opaque layer.